This invention relates to photoflash lamps and, more particularly, to flashlamps containing a combustible material which is ignited to produce actinic light.
A typical photoflash lamp comprises an hermetically sealed glass envelope containing a quantity of combustible metal, such as shredded zirconium or hafnium foil, and a combustion-supporting gas, such as oxygen, at a pressure well above one atmosphere. In lamps intended for battery operated flash systems, the envelope also includes an electrical ignition system comprising a tungsten filament supported on a pair of lead-in wires having a quantity of ignition paste on the inner ends thereof adjacent to the filament. This type of lamp is operated by the passage of an electrical current through the lead-in wires which incandesces the filament to ignite the ignition paste which in turn ignites the combustible metal in the envelope. In the case of percussive-type photoflash lamps, such as described in U.S. Pat. No. 3,535,063, a mechanical primer is sealed in one end of the lamp envelope. The primer may comprise a metal tube extending from the lamp envelope and a charge of fulminating material on an anvil wire supported in the tube. Operation of the percussive photoflash lamp is initiated by an impact onto the tube to cause deflagration of the fulminating material up through the tube to ignite the combustible metal disposed in the lamp envelope.
Typically, the flashlamp envelope is comprised of G-1 type soft glass having a coefficient of thermal expansion within the range of 85 to 95 .times. 10.sup.-.sup.7 in./in./.degree. C between 20.degree. C. and 300.degree. C., and the metal from which the primer tube is formed or the lead-in wires are made has a similar coefficient of thermal expansion so as to provide a match seal.
During lamp flashing, the glass envelope is subject to severe thermal shock due to hot globules of metal oxide and/or molten metal impinging on the walls of the lamp. As a result, cracks and crazes occur in the glass and, at higher internal pressures, containment failure becomes possible. In order to reinforce the glass envelope and improve its containment capability, it has been common practice to apply a protective lacquer coating on the lamp envelope by means of a dip process. To build up the desired coating thickness, the glass envelope is generally dipped a number of times into a lacquer solution containing a solvent and a selected resin, typically cellulose acetate. After each dip, the lamp is dried to evaporate the solvent and leave the desired coating of cellulose acetate, or whatever other plastic resin is employed.
In the continuing effort to improve light output, higher performance flashlamps have been developed which contain higher combustible fill weights per unit of internal envelope volume, along with higher fill gas pressure. In addition, the combustible material may be one of the hotter burning types, such as hafnium. Such lamps, upon flashing, appear to subject the glass envelopes to more intense thermal shock effects, and thus require stronger containment vessels. One approach to this problem has been to employ a hard glass envelope, such as the borosilicate glass envelope described in U.S. Pat. No. 3,506,385 of Weber et al, along with a protective dip coating of cellulose acetate. More specifically, the Weber patent describes an electrically ignitable lamp having in-leads of a metal alloy such as Rodar or Kovar secured by an internal expansion match seal in a borosilicate glass envelope having a coefficient of thermal expansion in the range of 40 to 50 .times. 10.sup.-.sup.7 in./in./.degree. C. Type 7052 glass, having an expansion of about 47, is mentioned as typical. It is theorized that glass in this thermal expansion range provides a more beneficial mode of fracture which results in a delay in crack time after flashing to a point where containment is more readily assured due to the reduction in lamp pressure over than period. In the hypothesis set forth in column 4, lines 56-65 of the Weber patent, this thermal expansion range is treated as relatively critical. The coefficient of thermal expansion is to be high enough to cause substantial amounts of shaling of the inner surface of the lamp envelope at points of molten droplet impingement, so as to relieve the thermal and mechanical stresses in the glass, but not so high as to cause excessive deleterious crack propagation penetrating through the lamp wall. The patent teaches a maximum expansion coefficient of about 50, with an example in column 3, line 40, indicating a figure of 51 for type 706.times.1 glass.
In attempting to use such a glass envelope in the above-described percussive lamp structure, however, we have encountered sealing problems leading us to conclude that the coefficient of thermal expansion of the commercially suitable metals for the primer tube are not sufficiently low enough to provide a good, consistently crack-free seal to the glass. More specifically, referring to the envelope primer assembly of FIG. 1, in the above-referenced prior art percussive flashlamp, the metal primer tube 10 is secured to the glass envelope 12 by means of an internal expansion match seal. When envelope 12 is formed of a glass of the type described in the Weber patent (having a mean coefficient of thermal expansion about in the range of 40 to 50 .times. 10.sup.-.sup.7 in./in./.degree. C. between 0.degree. C. and 300.degree. C.) and tube 10 is formed of a low thermal expansion metal alloy such as Kovar or Rodar (having a mean coefficient of thermal expansion of about 50 .times. 10.sup.-.sup.7 in./in./.degree. C. between 25.degree. C. and 300.degree. C.), there appears to be a mismatch between the mating materials at the set point of the glass, such that upon cooling, the Kovar contracts more than the glass. In this event, tube 10 continues to adhere to envelope 12, but the greater contraction of the metal places the adjacent glass area under tension, as illustrated by the arrows. This results in an unacceptably weak seal area, as the strength of the glass is reduced in tension. For example, upon examining test samples of percussive-flashlamps made with envelopes 12 of one of the preferred glass compositions of the Weber patent, namely type 7052 glass (expansion of 47), we have observed cracks in the glass at the edge of the primer tube flare 10.
U.S. Pat. No. 3,832,124, assigned to the present assignee, confronts a similar seal mismatch problem with respect to an even lower expansion glass composition by employing a special primer tube with a tubular rim which bears against the exterior surface of the glass envelope, whereby the glass envelope is placed under compression upon cooling from the sealing process. Under a compressive strain, glass is made considerably stronger; hence, even though the materials are mismatched, a strong seal results. A disadvantage of such a solution, however, is that a special, somewhat more complexly shaped primer tube is required, thereby imposing added cost.
Another approach to the problem is described in U.S. Pat. No. 3,771,941, wherein a graded seal is employed comprising a bead of intermediate expansion glass sealed between a lower expansion glass envelope and a higher expansion primer tube. More specifically, a doughnut-shaped preform (bead) of pressed and sintered glass powder, such as type 7050 glass having an expansion of 46, is sealed about a Kovar or Rodar primer tube (expansion of 50) of the standard type shown in FIG. 1; then the end of a tubular envelope of, say, type 7070 hard glass (expansion of 32) is sealed about the glass bead. The resulting graded seal, even though involving a thermal expansion differential between the bead and primer tube similar to that between the envelope 12 and tube 10 of the lamp described with respect to FIG. 1, appeared to avoid the creation of high stresses at the glass to metal interface and cracking of the seal. This is probably due to lower contraction differences between the sealed components and the greater amount of working required to provide a graded seal. Disadvantages of this approach, however, are the added manufacturing cost involved in the requirement of a preformed glass bead in addition to the glass envelope and the greater amount of processing required to provide a graded seal.